Some industrial processes, such as those in the petroleum industry, require devices to operate in hazardous atmospheres or other dangerous conditions. Such devices are often governed by a standard of “intrinsic safety”, such as the ISA-60079-11 standard, specifying certain conditions for devices in hazardous atmospheres. The conditions seek to either limit the amount of energy stored in device circuitry (e.g., by limiting voltages across capacitors or currents through inductors) or limit the discharge of accumulated energy (e.g., by restricting the spacing of components) such that a discharge of energy will not cause an ignition. In the case of the ISA-60079-11 standard, certain attributes of a device can be considered “infallible” if certain conditions, prescribed in the standard, are met. Generally, the circuitry of a device is considered “Intrinsically safe” if the circuitry remains safe with up to two “countable” faults.
Commonly, device circuits use Zener diodes as shunt voltage limiters to limit the voltage that could reach certain device components. However, while effective at such voltage limiting, Zener diodes must be operated at a voltage well below the Zener voltage to prevent leakage of current near the Zener voltage. In power sensitive applications (e.g., instruments powered by 4-20 mA signals), such current leakage is a significant concern. On the other hand, if a higher-voltage Zener diode is properly specified to prevent current leakage, the voltage limit increases, which may cause issues with respect to intrinsic safety.